For production of tires and other vulcanized rubber goods, it is desirable to provide uncured rubber compositions with relatively low viscosity in order to more efficiently speed manufacturing processes such as, for example, rubber extrusion, rubber calendaring, rubber milling, injection and mold shaping, before the rubber product is in its final state for vulcanization.
However, a negative aspect of lowering the viscosity of the formative uncured rubber composition is that the stiffness, or modulus, of the finally cured rubber composition may also be lowered which may be undesirable for many rubber products.
For this invention it is desired to evaluate whether processing an uncured rubber composition at a relatively lower uncured viscosity (uncured G′ value), or by substantially maintaining the relatively low viscosity, such as for example during its high shear mixing, or processing, followed by providing followed by providing the sulfur cured rubber composition with a suitable stiffness, or cured G′ value can be accomplished.
For such evaluation, a two step process is envisioned.
For the first step, use of a controlled free radical agent (CFR) is contemplated for use in providing a relatively low viscosity (Mooney viscosity) of the uncured rubber composition after its mixing and during its subsequent processing such as, for example, high shear mixing, or processing.
While the mechanism may not be completely understood, for such first step, it is envisioned that free radicals are formed on the uncured elastomer, particularly for natural cis 1,4-polyisoprene rubber, during its mixing process (with other compounding ingredients) under high shear conditions at an elevated temperature such as, for example, at a temperature in a range of from about 140° C. to about 170° C. As the rubber composition cools, such as for example to a temperature below 120° C., an added CFR agent may have an ability to react and form reversible chemical bonds with the radicals which had been formed on the elastomer polymer chains during the mixing process.
It is envisioned that such phenomenon of tying up the formed free radicals on the elastomer chain by the CFR agent prevents or retards a viscosity build up of the uncured rubber composition.
Later, however, as the elastomer composition is cured, or vulcanized, an elevated temperature, such as for example above 120° C., and perhaps in a range of from about 140° C. to about 170° C., the CFR agent is envisioned as being able to break away from the elastomer chain in a sense that the aforesaid reversible chemical bonds between the CFR and created free radicals on the elastomer's polymer chain are broken to thereby leave free radicals on the elastomer's polymer chain.
For this evaluation and invention, it is envisioned that the controlled free radical agent (CFR agent) may be, for example, a chemical compound containing a nitroxide free radical such as, for example, 2,2,6,6-tetramethylpiperidine-1,4-diol (4-hydroxyTEMPO) and derivatives thereof so long as said derivatives contain nitroxide free radicals and 2,2,5,5-tetramethyl-1-pyrrolidinyloxy (PROXYL) and derivatives thereof so long as said derivatives contain nitroxide free radicals which are reactive with the aforesaid free radicals created on the elastomer's polymer chain. An exemplary reference may be found as G. Moad, et al, “Tetrahedron Letters”, Volume 22, Page 1165 (1981) which refers to free radical polymerization inhibitors.
Historically, 4-hydroxyTEMPO has been used as a free radical scavenger and in various rubber compositions. For example, see U.S. Pat. Nos. 3,334,103; 6,084,015; and 6,194,509; U.S. Application Publication Nos. 2008/0051496; 2008/0085973; and 2009/0069469 as well as literature references “Mechanisms of Antioxidant Action”, L. P. Nethsinghe, et al, University of Aston in Birmingham, England, received Jan. 15, 1984, Rubber Chemistry and Technology, Volume 57, Pages. 779 through 791; “Mechanisms of Antioxidant Action”, H. S. Dweik, et al, University of Aston in Birmingham, England, received Dec. 2, 1983, Rubber Chemistry and Technology, Volume 57, Pages 908 through 917; “Mechanisms of Antioxidant Action” H. S. Scott, et al, received Apr. 20, 1983; Rubber Chemistry and Technology, Volume 57, Pages 735 through 743.
For the second step of evaluation, a free radical reactive compound (referred to herein as “FRRC”) is added to the CFR treated rubber composition (thereby added subsequent to the CFR treatment and in a later, and lower temperature mixing stage, such as a productive mixing stage where sulfur curatives are added, at a temperature in a range of, for example, from about 90° C. to about 115° C.).
Such free radical reactive compounds (FRRC's) have heretofore sometimes been used as curative co-agents when used in combination with various organoperoxides for curing various organoperoxide curable rubber compositions. Such FRRC's have been thought to increase cure rates as well as crosslink density of the various rubber compositions during the curing of the peroxide curable rubber compositions with various organoperoxides. However, for this evaluation, the FRRC agent is to be used exclusive of an organoperoxide.
For this experiment, the FRRC's are evaluated for use in reacting with free radicals formed on the sulfur curable elastomer chain by liberation of the CFR material from the elastomer chain at an elevated temperature above perhaps 120° C. which would be experienced during sulfur vulcanization of the rubber composition. In such manner, then, the added FRRC becomes attached to the elastomer's polymer chain in place of the CFR material as it reacts with free radicals on the elastomer's polymer chain left by the liberation of the CFR material. It is envisioned that the attached FRRC promotes linkages between the polymer chains to significantly increase the molecular weight of the elastomer and to thereby increase various physical properties of the finally sulfur cured rubber composition.
Representative of various FRRC's are, for example, zinc acrylates, di-acrylates and tri-acrylates; triallyl cyanurates; triallyl phosphates; low molecular weight high vinyl polybutadiene resins, having, for example, a vinyl content in a range of from about 40 to about 80 percent, which are considered as being reactive with the aforesaid free radicals on the elastomer chain.
Representative examples of such FRRC's are, for example, zinc dimethacrylate, zinc methacrylate, trimethylolpropane trimethyacrylate, triallyl cyanurate, triallyl phosphate and low molecular weight 1,2-vinyl polybutadiene resin having a vinyl content in a range of from about 40 to about 80 percent.
As indicated, it is envisioned that the presence of the FRRC's results in the ability of the very reactive radicals formed on the polymer chain of the elastomer, upon release of the CFR agent, to react with and combine several of the polymer chains together thereby resulting in a relatively dramatic increase in the effective molecular weight of the elastomer and thereby the modulus of the cured rubber composition to an extent not normally possible through traditional rubber processing and vulcanization methods.
For this invention, then, it is proposed to evaluate providing a more easily processable uncured rubber composition (e.g. more easily extruded) by lowering its viscosity, or substantially maintaining a lower rubber viscosity (e.g. its uncured rubber G′ value) for rubber processing purposes and subsequently provide a relatively high modulus cured rubber composition (relatively high cured rubber G′ value). Such rubber composition might be used, for example, for tire components where a stiff rubber composition may be used for an advantage, such as, for example, internal tire sidewall components such as for example apex and sidewall stiffening component spaced apart from a tire sidewall apex; coating for metal wire tire cord bead; coating for wire tire cord circumferential belt positioned between the tire tread and tire carcass; and tire tread.
Accordingly, it is an aspect of this invention to undertake such evaluation.
In the description of this invention, the terms “rubber” and “elastomer” may be used interchangeably unless otherwise indicated. The terms “rubber composition”, “compounded rubber” and “rubber compound”, may be used interchangeably to refer to “rubber which has been blended or mixed with various ingredients” and the term “compound” relates to a “rubber composition” unless otherwise indicated. Such terms are well known to those having skill in the rubber mixing or rubber compounding art.
In the description of this invention, the term “phr” refers to parts of a respective material per 100 parts by weight of rubber, or elastomer. The terms “cure” and “vulcanize” may be used interchangeably unless otherwise indicated.